1. Field of the Invention
The invention concerns a helical compression spring wound of wire of circular cross section with a partially progressive characteristic and especially suitable for use in motor vehicle construction, as a chassis spring, for example.
2. Discussion of the Prior Art
Cylindrical helical compression springs having partially progressive characteristics and optimum material utilization are known. The optimum material utilization is achieved by the fact that the springs have an inconstant wire diameter which makes possible a constant material stress in all coils.
One disadvantage of the cylindrical helical compression spring consists in the fact that the coils which have to be flattened successively for the achievement of a progressive characteristic pile up on one another and consequently cause noise. For prevention of the noise, plastic tubing is often drawn over the stacked coils, and this, of course, increases the cost of the springs. Another disadvantage of the cylindrical helical compression spring is to be seen in the fact that the ratio of the largest to the smallest wire diameter is relatively great, as will be shown in greater detail below, and this greatly increases the manufacturing costs, whether the wire diameter is reduced by non-cutting methods or by cutting methods, such as spin drawing or paring.
Single and double truncoconical springs having a partially progressive characteristic are also known, in which the coils which have to be flattened to produce the progressive characteristic lie spirally within one another as the load increases, without touching one another. In the case of these springs, therefore, no noise is produced. However, a severe disadvantage of single and double truncoconical springs is that the ratio of the greatest to the smallest wire diameter is even greater than it is in cylindrical springs, as will be shown more precisely further below.
One common disadvantage of all helical compression springs wound from wire consists in the fact that the pressure center of the spring does not coincide with the geometrical center of the spring, but lies outside of the geometrical center. The result is that the spring exercises a torque on its support when loaded, the magnitude of which is determined by the distance between the two above-named centers.
In the case of the cylindrical helical compression spring, the effective coil diameter which transmits its pressure to the support is constant. As a result, the distance between the geometrical center and the pressure center remains substantially constant. In the case of a single or double truncoconical spring of progressive characteristic, however, this effective coil diameter varies, and does so as the loading increases. As a result, the pressure center migrates outwardly, i.e., away from the geometrical center, as the load increases. The distance between the two centers thus becomes greater as the loading changes, and accordingly the torque exercised on the support increases greatly.
Finally, another disadvantage of single or double truncoconical springs is to be seen in the fact that, for the same characteristic, they have a greater outside diameter than the cylindrical spring, which precludes the use of such springs in existing constructions. Double truncoconical springs furthermore have manufacturing disadvantages, since they cannot simply be wound on a single mandrel.